How Much Dark Matter Do Some Galaxies Need? 300 Billion Suns

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Creating stars is a lot like cooking: You need the right
ingredients in the proper proportions to make everything shine.
One of those ingredients is dark matter, and a new study has
pinpointed the lower limit of this elusive substance needed to
ignite a frenzy of star formation: a mass equal to 300 billion of
our suns.

Dark matter is an invisible substance that astronomers can
measure only indirectly by its gravitational influence over
regular, visible matter. But while it has yet to be directly
observed, it's a vital ingredient for galaxies in the act of
forming stars.

And if 300 billion solar masses' worth of dark
matter sounds like a lot to start with, scientists say it is
actually about 10 times less than the amount previously
estimated.

"If you start with too little dark matter, then a developing
galaxy would peter out," said the study's leader, Asantha Cooray
of the University of California, Irvine, in a statement. "If you
have too much, then gas doesn't cool efficiently to form one
large galaxy, and you end up with lots of smaller galaxies. But
if you have the just the right amount of dark matter, then a
galaxy bursting with stars will pop out."

The scientists studied a patch of sky the size of Earth's moon in
the constellation Ursa Major to make their discovery.

This wedge of sky, known as the Lockman Hole, is ideal for
studying objects outside our galaxy because of the low dust
contamination from the Milky Way.

The study's findings are detailed in today's (Feb. 16) online
edition of the journal Nature and will appear in print Feb. 24.

Active galaxies show the way

Star formation is especially high within so-called
submillimeter galaxies, which are some of the most active
stellar cradles in the 13.7-billion-year-old universe. (The
galaxies get their name from the emissions we detect from them as
they rapidly move away from Earth. The wavelength of the
emissions is less than a millimeter long.) In these old,
bright galaxies, new stars are created at the rate of up to a few
thousand per year. By comparison, the Milky Way produces about 10
stars annually.

"These are the galaxies that formed when the universe was about 2
or 3 billion years old," Cooray said.

After measuring the brightness of the galaxies within the patch
of sky, the researcher calculated the minimum dark halo mass
needed to develop and sustain a submillimeter galaxy when the
universe's star formation was at its peak.

That number, 300 billion solar masses, is substantially less than
previous estimates.

"There could be many reasons for this," Cooray said. For example,
"it could be that there are more galaxies in the universe
actively undergoing star formation than assumed by current
simulations."

Whatever the cause, the link between halo mass and star formation
will require another look at current theoretical models for these
ancient star-forming galaxies, as well as galaxy formation and
evolution as a whole, researchers said.